Surgical distraction device

ABSTRACT

A surgical distraction device is provided for applying extending or tensioning force non-invasively to a patient&#39;s skeleton or to an implant which comprises anchoring means for attaching first and second components of the device to a bone or to adjoining bones, said components being connected by a linkage of an extendible length, a magnet connected to the linkage via a reduction gearbox and actuating means located externally of the patient for generating a moving or varying electro-magnetic field, thereby causing the magnet to rotate and the linkage to be extended.

This invention relates to surgical distraction devices and in one formprovides a surgical distraction device for exerting an extending ortensioning force to bones and/or implants within a patient's skeleton.

In the treatment of malignant bone cancer it is often necessary tosurgically resect part of a bone and to replace it with a prosthesis.Where the patient is a child or adolescent, the skeleton continues togrow and in order to accommodate this the prosthesis requiresreplacement at intervals. In an effort to reduce the number of revisionoperations, procedures have been developed for extending the prosthesisin situ. Methods of achieving this include the provision of telescopicparts in the prosthesis which are extended by introducing spacers ofapproximate size between the parts. Devices which have been used forthis purpose are described in the paper by Schindler et al, the Journalof Bone and Joint Surgery, Volume 79b, No. 6, November 1997, pages 927to 937.

While the system described in the above paper is preferable torepetitive revision operations, it still requires surgical access to apart of the prosthesis at intervals which can be frequent, depending onthe growth rate of the patient. Since all operations carry finite risks,including the risk of introducing an infection, it is one object of thisinvention to provide a device which can be used to extend a prosthesisnon-invasively.

According to one aspect of the present invention there is provided asurgical distraction device for applying an extending or tensioningforce non-invasively to a patient's skeleton which comprises anchoringmeans for attaching first and second components of the device to a boneor to adjoining bones, said components being connected by a linkage ofvariable length, a magnet connected to the linkage via a reductiongearbox and means located externally of the patient for generating amoving or varying electromagnetic field, thereby causing the magnet torotate and the linkage to be varied in length.

In practice, the first and second components and its associated linkage,together with the magnet, are surgically introduced and anchored to thebone or bones, for example when fitting a prosthesis or implant.

At intervals when the device requires extension or tensioning, a movingelectric field is established around the patient's body in the region ofthe prosthesis or implant, thereby causing the two components to bepressed apart. Normally, the magnetic field is arranged to rotate in thedirection of desired movement of the magnet connected to the linkage.

According to a second aspect of the invention there is provided anextendable endoprosthetic replacement device which comprises first andsecond components which are connected by a linkage of extendable length,the first component comprising a component of a limb joint and thesecond component having means for attachment to a resected long bone, amagnet connected to the linkage and means adapted for locationexternally of the patient for generating a moving electromagnetic field,thereby causing the magnet to rotate and the linkage to be extended.

The distraction device in accordance with the invention has substantialadvantages over existing practices such as described in the paper citedabove, since it avoids the trauma of further surgical operations,reducing the risk of infection and the introduction of further scartissue as a result of the operations. Moreover, the device can beextended or tensioned over a longer period than might be practical inthe course of a surgical operation. The device can also be extended moregradually and more frequently, which induces an expansion or tensileforce, which keeps pace with growth and stimulates biological growth oftissues.

The provision of a reduction gearbox between the magnet and therelatively movable components of the distraction device carries with itimportant advantages. First it enables a uniform extension force to beapplied to the extensible components. Secondly it enables sufficientforce to be applied to overcome resistance to extension and to resistany tendency for the device to contract in length when the bone isloaded.

Although a major use of the device in accordance with the invention isthe periodic extension of prostheses for the reasons described in thepaper cited above, there are a number of other surgical purposes forwhich the device is of value. One of these is in distractionosteo-genesis. This treatment involves tensioning of bone in order tostimulate its growth The conventional procedure involves provision of anexternal frame, e.g. around a limb and pins are anchored to the bone ata spaced distance. Tensioning rods are then fitted between the pins inorder to apply a constant tensioning force to the bone. These devicesare extremely cumbersome and awkward for the patient, often making itdifficult or impossible for the patient to walk. In addition, there is aconsiderable risk of infection caused by the use of transcutaneous pins.With the device of the present invention, it can be surgicallyintroduced and the components anchored to the bone and tensioning can beapplied either continuously or at intervals by placing the means forgenerating the rotating electromagnetic field around the implanteddevice.

The device also has application for curing curvature of the spine(scoliosis). In this procedure, the components of the device will beanchored to appropriate vertebrae or to a rib and selected vertebrae.The distraction force is applied using an externally generatedelectro-magnetic field to overcome the resistance of the tissue of thelimb, together with the friction of the linkage. Use of the device isnot limited to applying distraction forces to bone but could, in amodified form, be used for attachment to tendons and thereby used, e.g.for treating cerebral palsy.

According to a further aspect, therefore, there is provided a surgicaldistraction device for correcting curvature of the spine which comprisesfirst and second extension rods, each having attachment means at one endfor fixing to a respective vertebra, a linkage for linking the rods sothat the ends carrying attachment means are relatively moveable, saidlinkage being connected to a drive mechanism comprising a rotatablemagnet and a reduction gearbox and actuating means for locationexternally of the patient for generating a moving or varyingelectric/magnetic field, thereby causing the magnet to rotate and theattachment means on one rod to move relatively to the attachment meanson the other rod.

One embodiment of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of the distraction device fitted to a femurand forming part of a femoral prosthesis;

FIG. 2 is an enlarged, diagrammatic view showing the linkage between thetwo relatively extendible parts of the prosthesis;

FIG. 2 a is a view similar to FIG. 2 but showing the linkage in moredetail;

FIG. 3 is an exploded view of the gearbox;

FIG. 4 illustrates an alternative method of fixing the prosthesis to abone; and

FIG. 5 is a perspective view of another embodiment of a distractiondevice for correcting curvature of the spine.

Referring to the drawings, the distraction device comprises twocomponents, a first component (1) forming part of a femoral hipprosthesis including a ball joint (3), forming an artificial hip jointand a relatively moveable component (2) carrying a spigot (4) foranchorage to the re-sected femur (5). The components (1) and (2) arelinked together by a linkage comprising an elongated screw (6) and a nut(7). Relative movement of the linkage axially in the direction of thearrow X is achieved by a rotatable magnet (8) connected to a gearbox(9). Magnet (8) is connected through the gearbox (9) to the spindlescrew (6) and rotation of the magnet (8) causes rotation of the screwand hence elongation of the device.

Rotation of the magnet is achieved by placing an electromagnetic coil(10) around the limb and generating a moving electrical field. Themagnet is designed so that the moving electric field causes the magnetto rotate in the desired direction, to cause extension of the device,the magnet and the electromagnetic coil together constituting anelectrical motor. In general the electromagnetic coil takes the form ofa ring of coils which may be incorporated or embedded in a magneticallysusceptible material, e.g. soft iron, the ring being dimensioned to fitaround the limb which contains the extendable distraction device.

The gearbox is a high reduction ratio gearbox which may have a reductionratio in the order of several hundred, e.g. about 500. This ratio, thespeed of rotation of the magnet and the pitch of the screw is selectedso that, preferably, one turn of the rotor will advance the screw by afraction of a millimetre, e.g. 0.001 to 0.01 mm, typically about 0.005mm. The patient can remain within the electromagnetic coil for anextended period, e.g. one to two hours, depending upon the amount ofextension or length of distracting force required.

In a procedure for lengthening a prosthesis, one would typically wish toextend the prosthesis by 5 to 6 mm at each stage. The distraction forceshould be such that it is capable of overcoming friction within thelinkage and the resistance provided by soft tissue around theprosthesis.

FIG. 3 shows details of the gearbox. The gearbox incorporates twoplanetary gears and is sealed within a casing to exclude body fluids.The parts shown are all manufactured from stainless steel with theexception of the part (12) which is manufactured from titanium alloy andthe part (26) which is manufactured from a cobalt chrome alloy.

The gearbox and magnet assembly are designed to be housed within achamber (201) at the lower end of the linkage assembly (see FIG. 2 a).

Reference numeral (11) shows an axial spindle on which the drivingmagnet rotates. The lower end of the spindle (11) is formed with apointed portion so that the spindle is guided in a recess in the end cap(202—see FIG. 2 a). Part (12) is a housing for containing the permanentmagnet (8). The permanent magnet is preferably manufactured by asintering process, has an annular shape and is fitted into aninterlocking annular capsule (12) to ensure that the magnet will notrotate within the capsule.

After welding into the capsule (12) the magnet is magnetized with afield direction parallel to the diameter of the capsule (12). Part (13)is the input pinion to the first stage of the gearbox. The pinion isbored to be a free running fit on the spindle (1) and has a plane bossthat is interlocked into the capsule (12). The components (14) and (17)are fitted together as a spigot and socket to form a planetary carrierassembly and planet gears (16) are fitted into the carrier assembly.Each gear stage has three identical planet gears which are fitted intothe carrier assembly and each planetary gear is machined with a smallintegral stub shaft at each end. These stub shafts locate in holes ineach half of the carrier assembly.

Components (15) and (22) are fixed internal gears for the first andsecond reduction stages respectively. These gears each mesh withapproximately half of the face width of a set of planetary gears and arefixed into the body of a gearbox. The remaining half of the planetarygears is meshed with rotating internal gear (18). A reduction of thenumber of teeth in the rotating internal gear (18) compared with thenumber of teeth in the fixed internal gear (15) provides speed reductionfor the stage. Higher number of reductions in the number of teethprovide a greater reduction using the minimum number of gears and space.Components (18), (19) and (20) comprise an assembly which transfers theoutput from the first stage of the gearing to the input of the second.The assembly comprises a gear web part (19) which is welded to both therotating internal gear of the first stage of gearing and the inputpinion of the second stage.

Parts (21), (22), (23), (24) and (25) comprise the fixed internal gearand planetary gear and carrier assembly for the second stage. Theseparts are the same as parts (14), (15), (16), (17) and (18) of the firststage of gearing.

Part (25) is the rotating internal gear of the second stage of thegearing and this is welded to part (26) which is the output shaft fromthe gearbox. This passes through O-ring seals (203) to drive the screw(6) shown in FIGS. 1, 2 and 2 a.

Part (26) is a spigot fit into a socket machined into part (25) so thatthis assembly is self-jigging when set up for welding. This is the samearrangement as in the case of parts (18) and (19).

Part (26) is shown with a short spigot that locates in a bore in oneside of the carrier (24). The purpose of this spigot is to ensure thatthe planet carrier is accurately centred in the mechanism and a similarspigot is provided on part (19) to centre the first stage planetcarrier.

The upper face of part (26) carries an output shaft (27) whichterminates in a suitably shaped drive projection engaging in a leadscrew (6). Screw (6) is threadably received within an inner shaft (7).Outer shaft (2) surrounds inner shaft (7) so that inner shaft isextended out of shaft (2) when lead screw (6) is rotated by the magnet(8) driving through the reduction gearbox Inner shaft (7) is preventedfrom rotating by means of a key (204) which is fixed into acorresponding aperture in the outer shaft (7). Key (204) has aprojecting portion which extends into a longitudinal slot (205) formedin the inner shaft (7).

The device may include an integral or separate sensor which counts thenumber of revolutions of the magnet (8) so that the degree of extensionof the device can be monitored and/or recorded.

FIG. 4 illustrates an alternative method of anchoring one component ofthe device to a long bone, which has been found to result in improvedstability and bone integration. As can be seen, the tubular body (2) ofthe prosthesis is fixed to a resected bone (5) by means of two or moreplate-like strips (401) extending from the tubular body. The plate-likestrips are apertured to receive unicortical screws (402) for fixing theplate-like strips to the outer surface of the bone. The strips wereformed with elongated holes (403) to encourage bone ingrowth and coatedwith a highly crystalline hydroxyapatite coating to stimulateintegration with the bone.

FIG. 5 shows a distraction device suitable for treatment of curvature ofthe spine. In this embodiment, threaded rods (501) and (502) areprovided with attachment means (503), such as plates or blocks carryingscrews or hooks. Depending on the curvature of the spine, the threadedrods (501) may be straight or curved. The attachment means may include auniversal bearing or joint connection to the threaded rods so that theattachment means can be rotated or swivelled to take account of thecurvature or deformation of the spine, and to enable firm attachments tobe made to appropriate vertebrae.

The two threaded rods are joined by a central spacer or block (504). Theblock (504) may be stabilised by attachment to the spine, e.g. usingscrews or hooks. Block (504) includes a drive mechanism comprising arotatable magnet and reduction gearbox as described in FIGS. 1 to 3.However, the final drive component comprises a cog carried by the outputshaft. The cog is linked to two worm drives which are turned by the cog.In one arrangement, the worms are arranged in series such that they aredriven in opposite directions. In another arrangement, the worms may bearranged to be driven in the same direction. Single worm or multipleworm drives of two or more worms may also be used to link the cog to thethreaded rods. Each worm is cylindrical and has a central tapping. Acorresponding threaded rod (502) is received within the worm and thethreads on the rod match the tapping on the inside of the worm. As theworm is turned by the cog, the threaded rod is also turned and is movedin a direction lengthwise of the rod. The direction of movement of therod depends on the sense of the cooperating threads and on thearrangement of the worms and gears which transmit the rotation of themagnet. In a similar way to the arrangement described in connection withFIGS. 1 to 4, a rotating electrical field is generated by means of anexternal annular coil around the patients' spinal region.

1. A surgical distraction device for applying an extending or tensioningforce non-invasively to a patient's skeleton or to an implant whichcomprises anchoring means for attaching first and second components ofthe device to a bone or to adjoining bones, said components beingconnected by a linkage of an extendable length, a magnet connected tothe linkage via a reduction gearbox having two or more gear reductionstages whereby the gearbox has a gear ratio of 100:1 or greater andactuating means located externally of the patient for generating amoving or varying electro-magnetic field, thereby causing the magnet torotate and the linkage to be extended.
 2. A device according to claim 1,wherein the actuating means is adapted to generate a rotating magneticfield which rotates in the desired direction of rotation of the magnet.3. A device according to claim 1, wherein the linkage comprises a screwand nut and the magnet is connected to the screw or nut via the gearboxto cause relative rotations, thereby extending the device.
 4. A deviceaccording to claim 1, wherein there is a difference in the number ofteeth between gears, thereby providing a reduction in the number ofgears and space.
 5. A device according to claim 1, wherein the gearboxhas a gear ratio of 100:1 to 1000:1.
 6. A device according to claim 1,wherein the two components are components of a prosthesis for fixing toa long bone.
 7. A device according to claim 6, wherein the long bone isa femur.
 8. An extendable endoprosthetic replacement device whichcomprises first and second components which are connected by a linkageof extendable length, the first component comprising a component of alimb joint and the second component having means for attachment to aresected long bone, a magnet connected to the linkage and means adaptedfor location externally of the patient for generating a movingelectromagnetic field, thereby causing the magnet to rotate and thelinkage to be extended wherein the magnet is connected to the linkagevia a reduction gearbox having two or more gear reduction stages wherebythe gearbox has a gear ratio of 100:1 or greater.
 9. A device accordingto claim 8, wherein the first component comprises a component of a hipor knee joint.
 10. A device according to claim 9, wherein the secondcomponent comprises a spigot adapted for fixing into the intermediarycanal of a long bone.
 11. A surgical distraction device for correctingcurvature of the spine which comprises first and second extension rods,each having attachment means at one end for fixing to a respectivevertebra, a linkage for linking the rods so that the ends carryingattachment means are relatively moveable, said linkage being connectedto a drive mechanism comprising a rotatable magnet and a reductiongearbox having two or more gear reduction stages whereby the gearbox hasa gear ratio of 100:1 or greater and actuating means for locationexternally of the patient for generating a moving or varyingelectric/magnetic field, thereby causing the magnet to rotate and theattachment means on one rod to move relatively to the attachment meanson the other rod.
 12. A device according to claim 11, wherein theactuating means is arranged to generate a rotating field which rotatesin the desired direction of rotation of the magnet.
 13. A deviceaccording to claim 11, wherein the linkage comprises a drive cog whichengages with a worm encircling a rod so that movement of the worm causesthe rod to extend lengthwise thereof.